Study Shows Marijuana Really Does Lower IQ


A study now lays to rest the idea that marijuana use among teens is harmless and has no lasting effects. Reporting in the Proceedings of the National Academies of Sciences, the authors found a drop in IQ among those who began smoking regularly prior to age 18.

'Regularly', in this study, means daily use. The IQ drop measured was 8 points. This is a significant difference, although IQ does bounce around, depending on the test used and how a person is performing on any particular day. Participants who started using marijuana after the teen years did not show this effect. Marijuana advocates point out that legalization would still mean adult-only use, and the study actually supports the “no harm” position for adults.

Trend toward greater use among adolescents

The National Institute on Drug Abuse has been reporting a trend toward greater cannabis use in adolescents. The assumption is that partial legalization for medical purposes has changed perceptions of youth who now consider marijuana less dangerous.

The study looked at IQ scores from tests administered when participants were 13, before any marijuana use, and compared the scores with tests given at age 38. The IQ drop found can then be said to be persistent and the changes in the brain permanent. There might actually be damage occurring in adults, but the measures used in this test were not specific enough to tease them out.

At this point, just how marijuana use is affecting the brain to lower intelligence isn’t known. Researchers will have to discover what alterations are happening, perhaps even at the cellular level. The reason that age-specific use is significant may have to do with brain development. Human brains continue to mature until early adulthood, so any damage that happens before the brain is fully developed can be permanent.

More information about the study and its results can be found here.


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